A fuel cell is a cell for converting a reaction energy of a gas as a raw material directly into an electric energy, and a hydrogen-oxygen fuel cell has little adverse effect on the global environment because in principle, its reaction product is water only. Among such fuel cells, with recently increasing social needs for energy and global environment problems, polymer electrolyte fuel cells using a polymer membrane as an electrolyte are greatly expected to be used as a power source for movable bodies such as electric vehicles and for small cogeneration systems because it can operate at ambient temperature and obtain a high power density by virtue of development of a polymer electrolyte membrane having a high ion conductivity.
The polymer electrolyte fuel cells normally employs a proton-conductive ion exchange membrane as a polymer electrolyte and, particularly, an ion exchange membrane composed of a perfluorocarbon polymer having sulfonic acid groups is excellent in basic properties. In the polymer electrolyte fuel cells, gas diffusible electrode layers are disposed on both sides of the ion exchange membrane, and a gas containing hydrogen as a fuel and a gas containing oxygen as an oxidizing agent (air or the like) are supplied to an anode and to a cathode, respectively, to generate electric power.
A reduction reaction of oxygen on the cathode of the polymer electrolyte fuel cells proceeds via hydrogen peroxide (H2O2) and this arouses concern about a possibility that hydrogen peroxide or a peroxide radical produced in a catalyst layer induces deterioration of the electrolyte membrane. Likewise, there is also concern that hydrogen peroxide or the peroxide radical is produced in the anode because of permeation of oxygen molecules from the cathode through the membrane. Particularly, in a case where a hydrocarbon membrane is used as the polymer electrolyte membrane, it lacks stability to the radical, and it was a problem in long-term operation.
For example, the polymer electrolyte fuel cells that was first put into practice was the one adopted as a power source for Gemini spaceship of U.S.A. At this time, a membrane composed of a sulfonated styrene-divinylbenzene polymer was used as the electrolyte membrane, but there was a problem of insufficient long-term durability. The known technologies to remedy this problem include a method of adding into the polymer electrolyte membrane a transition metal oxide or a compound with a phenolic hydroxide group which can catalytically decompose hydrogen peroxide (cf. Patent Document 1) and a method of making metal particles of a catalyst supported into a polymer electrolyte membrane to decompose hydrogen peroxide (cf. Patent Document 2). However, these technologies were those of decomposing hydrogen peroxide produced but were not attempts to suppress decomposition of the ion exchange membrane itself. Therefore, they showed an initial effect of improvement but still could have a serious problem in the long-term durability. In addition, there was a problem of rise in cost.
On the other hand, there are known ion exchange membranes composed of perfluorocarbon polymers having sulfonic acid groups, as polymers having far greater stability to the radical, as compared with the hydrocarbon polymers. Recently, the polymer electrolyte fuel cells using the ion exchange membrane composed of these perfluorocarbon polymers are expected as power sources for vehicles, a residential market, and so on, and there are increasing demands for practical applications thereof, which accelerates development. Since these applications require operation in particularly high efficiency, there are desires for operation at a higher voltage and for cost reduction. Furthermore, the operation is often required under low humidification or no humidification from the viewpoint of the efficiency of the whole fuel cell system.
However, even in the case of the fuel cells using the ion exchange membrane of the perfluorocarbon polymers having sulfonic acid groups, it is reported that the stability is very high in operation under high humidification, whereas the voltage degradation is significant in the operating condition under low humidification or no humidification (cf. Non-patent Document 1). Namely, it is considered that, in the case of the ion exchange membranes of the perfluorocarbon polymers having sulfonic acid groups, deterioration of the electrolyte membrane also proceeds due to hydrogen peroxide or the peroxide radical in the operating condition under low humidification or no humidification.
Furthermore, processes for producing the electrolyte membrane for fuel cells as described above include a process for forming a film by extrusion, a process for forming a film by casting with the use of a solution of a resin constituting the electrolyte membrane, and so on. The cast film formation is effective for cases where a large-scale thin film is industrially produced. Moreover, it is also reported that a liquid composition containing a resin constituting a membrane such as the fluorocarbon polymer having sulfonic acid groups is used for production of a material for the membrane and is extremely useful for restoration, recovery and a coating material for a membrane already produced (for example, cf. Patent Document 3 and Patent Document 4).                Patent Document 1: JP-A-2001-118591 (claim 1, page 2, lines 2 to 9)        Patent Document 2: JP-A-6-103992 (Means for Solving the Problems, page 2, lines 33 to 37)        Patent Document 3: JP-A-2003-183467 (page 2, lines 15 to 32)        Patent Document 4: JP-A-2004-75978 (page 5, lines 24 to 41)        Non-patent Document 1: Summary of debrief session for polymer electrolyte fuel cells research and development achievement in 2000 sponsored by New Energy and Industrial Technology Development Organization, page 56, lines 16 to 24        